The present application claims priority form Japanese application JP 2004-015049 filed on Jan. 23, 2004, the content of which is hereby incorporated by reference into this application.
The present invention generally relates to a charged particle beam apparatus. More specifically, the present invention is directed to a charged particle beam apparatus capable of measuring actual image magnification of a material image and an image magnification shift of the apparatus in high precision, and also, capable of automatically calibrating image magnification of the apparatus.
In charged particle beam apparatus which are typically known as scanning electron microscopes (SEMs), charged particle beams which have been focused in narrow beams are scanned over materials so as to acquire desirable information (for instance, material images) from the materials. Since such charged particle beam apparatus are employed in order to measure pattern widths and film thicknesses of semiconductor devices, it is very important to maintain image magnification of these apparatus in high precision.
In order to measure an image magnification error, as indicated in FIG. 2A, while employing a reference material for image magnification which has a periodic structure whose pitch dimension (period) is already known as a nominal value, an enlarged image of this reference material is acquired. Then, a pitch dimension of a material is measured based upon the acquired enlarged image. Thereafter, a shift between this measurement value and the nominal pitch value is used as an image magnification error. In order to calibrate image magnification of an apparatus, an image magnification control parameter of this apparatus is adjusted in such a manner that this image magnification error becomes minimum. In a scanning beam microscope, this image magnification control parameter corresponds to a coefficient for determining a relationship between image magnification and a beam scanning width on a material.
In general, a nominal pitch value of a reference material for image magnification represents an averaged pitch value of a repetition pattern of this reference material for image magnification. As a result, in order to perform an image magnification calibration in high precision, pitches are measured at at least 10 different positions on the reference material, and then, a pitch measurement value of an image must be determined from an average value of these measured pitch values.
FIG. 3A and FIG. 3B indicate an example as to conventional pitch measuring methods. As indicated in FIG. 3A, in order to measure a pitch (Lm) of a reference material from a material image, a cursor is manually set to a position corresponding to the pitch on the material image which has been acquired as a digital image, and then, a total number of pixels (N) between the cursors is counted. Based upon this counted value (Nm) and a pixel size (Lp) of the material image, the pitch (Lm) is calculated by the following equation (1)Lm=Nm*Lp  (1)
As other pitch measuring methods, as indicated in FIG. 3B, the following measuring method has been practically used. That is, while a pitch measuring area of a material image is designated, a pattern edge portion is detected from a line profile between these pitch measuring areas, and then, a pitch dimension is automatically measured. This line profile corresponds to a distribution of pixel values (brightness) along either a horizontal line or a vertical line. On the other hand, a relationship between the pixel size (Lp) and image magnification (M) is defined as follows:Lp=Kp/(Np*M)  (2)
In this equation (2), symbol “Kp” indicates a display size of an image in order to correctly display image magnification, and symbol “Np” indicates a total number of pixels. For example, assuming now that a total pixel number of an acquired image is equal to 640×480 pixels and a display size of the image is equal to 128 mm×96 mm, the pixel size “Lp” in the image magnification of 10,000 power becomes 128 mm/(640×10,000)=20 nm. As a consequence, when the image magnification “M” contains an error, this error appears as an error of the pixel size “Lp” of the equation (2), so that an error may be produced in the pitch dimension (Lp) calculated in the equation (1).
Based upon the pitch measurement value (Lm) and the nominal pitch value (Ls) of the reference material which have been measured by the above-described measuring method, a shift “ΔM” of image magnification of the apparatus may be calculated by the following equation (3):ΔM=(Lm/Ls)−1  (3)In a scanning beam microscope (SEM), the below-mentioned relationship (4) between image magnification (M) and a beam scanning width (Lb) on a material is established as follows:M=Km/Lb  (4).An image magnification coefficient “Km” is employed so as to control image magnification (namely, to control beam scanning width) within a control program of an SEM apparatus. In such a case that the image magnification shift “ΔM” of the above-described equation (3) is present with respect to such an image magnification coefficient “Km” before being calibrated, if the below-mentioned equation (5) is established, then the image magnification of the SEM apparatus may be calibrated:M=Km*(Lm/Ls)/Lb  (5)In other words, such a process operation for converting the value of the image magnification coefficient used to control the SEM apparatus to “Km*(Lm/Ls)” corresponds to an image magnification calibration.
In a general-purpose scanning electron microscope (SEM) in which operating ranges as to accelerating voltages and WD are wide, it is technically difficult to control the image magnification in high precision over the entire operating area. As a consequence, the image magnification precision of this general-purpose SEM apparatus is set to ±10%. As a result, in order to perform observations and dimension measuring operations in higher precision while using such a general-purpose SEM apparatus, image magnification calibrations must be carried out with respect to each of apparatus use conditions such as accelerating voltages and WD.
Also, for example, JP-A-2000-323081 discloses such a technical idea of a transmission type electron microscope (TEM). That is, two sheets of different images are compared with each other so as to measure an image magnification error of the TEM apparatus.